We have pioneered the concept of providing two independent thermal sensors, such as bimetallic elements for example and advantageously snap-acting bimetallic elements, in an element protector unit specially though not exclusively adapted for use with water boiling vessels formed of synthetic plastics materials, with the two independent sensors in effect providing primary and secondary protection with the secondary protection providing a back-up operative in the event that the primary protection fails. Our British Patent Specification No. 2,117,568 and corresponding U.S. Pat. No. 4,539,468 discloses such an element protection unit adapted to provide two separate levels of protection against dry switch-on dry situations, the unit comprising two independent thermal sensors each controlling a separate pair of switching contacts and each in thermally conductive relationship with the heating element. Reference may be had to GB Patent No. 2,117,568 or to U.S. Pat. No. 4,539,468 for a fuller appreciation of the double protection concept.
The difficulty has arisen in seeking to apply the aforementioned double protection concept to the present day hot water jugs formed from synthetic plastics materials and with the thermal sensors comprised by bimetallic elements that the relatively tall and slender design of the jugs, to be contrasted with the fat and squat shape of most electric kettles, has led to the element itself being of relatively compact design with very little space available in the element head for accommodating the bimetallic elements which, as will be appreciated, should be maintained in close thermal contact with the element head if a sufficiently fast response to an overheating condition is to be assured. We have been addressing ourselves to the solution of this problem and the present invention concerns a solution which we have devised.